1. Field of the Invention
This invention relates to offset printing and more particularly to devices which hold an unexposed printing plate and negatives for exposure of the printing plate.
2. Description of the Prior Art
In offset printing, as with other printing methods, a great portion of the time and expense is spent on the fashioning of the printing plates. As is well-known, printing plates are constructed to selectively transfer ink to the material, such as paper which is being printed.
A common method by which printing plates are fashioned is to photograph the text of the subject matter to be printed to produce a negative. For example, should the text be printed material consisting of dark lettering on a white background, the negative produced will be opaque with transparent portions thereof corresponding to the lettering of the text. The negative is placed over a plate and then exposed to intense light. Due to the chemical makeup of the plate, the printing image on the plate is rendered grease receptive and water repellant, while the non-print areas are rendered water receptive and ink repellant, thereby enabling the plate to be used in a printing process to reproduce the text.
During exposure of the printing plate and negative to the intense light, it is important that the negative and printing plate be held tightly together. Any spaces between the negative and printing plate causes the light passing through the transparent portions of the negative to diffuse which results in a printing plate which does not closely comport to the transparent portions of the negative and which results in fuzzy or ill-defined printing.
One known frame for supporting the plate and negatives during the exposure thereof provides a base having a hinged, glass cover. Extending across the base is an elastic surface, for example a rubber material, which is surrounded by a raised seal. When the glass cover is closed down on the base, the space between the glass and the elastic surface is sealed from the environment by the seal.
To fashion a plate, the plate is positioned and centered upon the elastic surface. The negatives, attached to a carrier sheet, are then positioned on top of the plate. The carrier sheet is opaque such as, for example, goldenrod paper, having the negatives secured thereto as by adhesive tape. Portions or "windows" are removed from the carrier sheet in the regions of the negatives so as not to interfere with the passage of light through the negatives to the plate.
After the carrier sheet and negatives have been positioned on the plate, masking material is superposed on the plate to, in combination with the carrier sheet, cover all portions of the plate not aligned beneath the negatives for exposure. The purposeof the masking material is to prevent unintended portions of the plate from being exposed.
After the plate, carrier sheet and mask have been positioned on the elastic surface, the glass cover is closed on top of the stack defined by the plate, carrier sheet, negatives and mask. A vacuum is induced into the vacuum space to evacuate air between the elastic surface and the glass. Evacuation of the air causes the elastic to press against the glass and hold the plate and carrier sheet and negatives and mask together for exposure of the plate.
A drawback of the frame set forth above is that it is relatively expensive to manufacture. The frame requires an elastic surface and means necessary to affix the surface to the base. Furthermore, the frame requires a seal around the elastic surface to permit the vacuum to hold the various layers tightly against the glass. Additionally, the frame requires a relatively strong sheet of glass and a framework necessary to support the glass since vacuums are typically in the range of from 27-28 inches of mercury below atmospheric pressure.
Another drawback of the frame described above is that the preparatory steps for exposing the plate are time-consuming. The carrier and mask must be fashioned for each set of negatives. In particular the attachment of the negatives to the mask and removal of the portions of the mask underlying the negatives consumes a great portion of time.
Another drawback concerns the production of plates for color printing. For color printing, negatives are produced for each color, the negatives, in turn, being used to fashion plates for that particular color. During printing, it is necessary that the plates of each individual color applying that particular color ink cooperate and coordinate with the other plates to produce the desired color printing. This coordination requires that the negatives be precisely positioned upon the plates so that when the completed plates are affixed to the printer rollers, the plates will coordinate to produce the desired result. Heretofore, the aligning and positioning of negatives on the carrier sheet and the carrier sheet on the plate was done by hand.
The present invention is directed toward overcoming one or more of the drawbacks noted above.